Xenobiologists and gaming enthusiasts alike know the intergalactic threat posed by the Tyranids. These voracious aliens wage a war of extermination against all organic lifeforms in an endless attempt to consume all biomass and incorporate all genetic codes.

But first they send in the Genestealers. These monstrosities plant gene-seeds in their victims, spawning devious cults of loyal hybrids to spread chaos across the soon-to-be-invaded world.

Of course you don’t have to leave Earth to find examples of gene theft. Various terrestrial creatures engage in this level of genetic cheating (or horizontal gene transfer). Here are just a few notable cases:

Asian clams: Strictly asexual, these hermaphroditic mollusks spice things up a bit to avoid genetic stagnation. And that means gene theft. While they generally fertilize their own eggs, they sometimes fertilize those of another clam species. This gives the resulting offspring an injection of fresh, alien genes, according to New Scientist.

Bdelloid rotifers: This all-female species of near-microscopic animals has been sex-free for 80 million years. But according to a 2012 University of Cambridge study, 10 percent of their expressed genes are pilfered from roughly 500 other species. They incorporate this foreign DNA (from fungi, plants, and bacteria) while patching up their own ruptured cell membranes. Read more over at Geekosystem.

Galdieria sulphurari: This single-celled red algae thrives in sun-lit hot springs but also manages in deep, dark depths. According to Live Science, the algae simply stole both genetic traits from simpler bacteria and archaea organisms.

Elysia chlorotica: If you ever see a sea slug with the power of photosynthesis, you can rest assure he/she stole it from some algae. That’s the story on this chlorophyll-producing mollusk. According to this Live Science article, the slugs even pass the chlorophyll-producing trait onto their offspring—though they to eat a bunch of algae to actually carry out photosynthesis.

Rafflesia cantleyi: Plants don’t only play the victim. This Malaysian parasite steals respiratory and metabolism genes from its host plant. In fact, as reported here in Science Daily, an entire third of Rafflesia cantleyi’s genes resemble those of the host.

Monster of the Week is a—you guessed it—regular look at the denizens of our monster-haunted world. In some of these, we’ll look at the possible science behind a creature of myth, movie or legend. Other times, we’ll just wax philosophic about the monster’s underlying meaning. After all, the word “monstrosity” originates from the Latin monstrare, which meant to show or illustrate a point.